ACROSS
 7. <illicit affair> DALLIANCE = _D ALLIANCE
 9. <volunteer> OFFER <hitman?>
11/12. <pretends not to see> TURNS A BLIND EYE = (INDUSTRY ENABLE)*
13. <German banker> RUHR = (R U HR)
14. <fascinated> MESMERISED = ME + (S + M) + ←DESIRE←
17. <one who might put people off> DEVIANT = VIA inside DENT
19. "RUE" BARB = RHUBARB <angry exchange>
21. AIRCON* outside ←JAMS← = CANIS MAJOR <stars>
22. <light-coloured> FAIR = F AIR
25/26. (FUN + EGG + WHISKY AT)* = GUY FAWKES NIGHT <annual celebration>
28. (.e)ARE NA(r.) = ARENA <concert venue>
29. <flee> CUT AND RUN = CU + TAN DRUN[-k]

DOWN
 1. (.o)ID IT A ROD(e.) = IDITAROD <contest in Alaska>
 2. <primeval> FIRST = FIRS + _T_
 3. (INNATE BIAS)* = IN ABSENTIA <while away>
 4/20. <carefully considering> DELIBERATING = (DELI BERATING)
 5. _N _D _E inside POROUS = PONDEROUS <weighty>
 6. <connection> FRIEND = FRIED outside N_
 8. <wood> LARCH = L ARCH
10. <plant> FREESIA = FREES IA
15. <one manipulated> MARIONETTE = (R + I ONE) inside MATTE
16. <military operation> FALSE FLAG = (ALL GAFFES)*
18. <hairstyle> VANDYKE = (V_ AND _Y) KE[-y]
21. <brandy> COGNAC = ←CAN GO C←
23. <fortune-teller> AUGUR = AU GUR[-u]
24. <pointless> INANE = A inside INN + _E
27. <messed up> WACK = "WHACK"